This paper reviews optical fibre sensors for industrial measurements, and covers mechanical, temperature, electrical, magnetic, nuclear and chemical sensors. It discusses industrial applications, operating problems, new sensor types and future needs.

A brief review is given of some STL work on optical flowmeters. These devices have involved the use of optical fibres with turbine or vane meters, vortex shedding and cross-correlation techniques. Assessments are made of these and future developments.

The paper describes the authors'motivation for developing miniaturized LDA-optical systems. Three readily developed systems, employing graded index fibers, are described in detail. Test measurements in laboratory flows are provided and an outline of future applications of the optical systems is given.

The use of a monomode optical fibre sensing element in the measurement of flow velocities by an interferometric technique is described. The operating principle is the flow induced oscillation of the fibre caused by vortex shedding.

For a fiber-optic LDV using an Ar-ion laser oscillated in about 40 longitudinal modes, enhancement of the Doppler signal above 20 dB was obtained when the fiber probes satisfied the phase-matching condition of all the longitudinal modes.

Fluctuations in the operating frequency of semiconductor lasers produce intensity noise in interferometric sensors with a path length difference. This noise has a 1/f frequency dependence and its magnitude increases in direct proportion to interferometer path length difference(1). An approximate value of the amplitude of these fluctuations, for the STL GaAlAs laser used to drive the magnetometer, is 105 Hz measured at a detection frequency of 1 kHz. To achieve a minimum detectable phase of 10-6 radians in a fibre interferometer with such a laser it is necessary to balance the interferometer arms to better than 1 mm. In order to reduce this noise source some authors(2) have reported locking the laser frequency to an external Fabry-Perot cavity by controlling the laser drive current. This is possible since the output frequency of a solid state laser is linearly dependent upon drive current. Compared with unstabilised lasers, improvements of 20 dB in laser output noise spectra have been reported using this technique. Similar results have been obtained in this work, but an all-fibre Fabry-Perot interferometer replaced the bulk optics device. This stabilised laser then acted as a source for a Mach-Zehnder sensing interferometer.

A closed loop d.c. field fibre optic magnetometer is described which utilises the dependence of the a.c. magnetostrictive responsitivity of a metallic glass sensing element on the local d.c. field in order to detect low frequency or d.c. changes in the total local field. A detection sensitivity of ti 1 μGauss has been demonstrated at frequencies below 2 Hz.

A highly sensitive and stable fiber-optic magnetic and electric field sensors using the multiple-reflection Faraday (SF6) and Pockels (BSO) cells are constructed and applied to the measurement of magnetic and electric fields in high-voltage electric systems.

Fiber optic magnetometers have been reported recently.1-3 These devices make use of magnetostrictive materials such as metallic glass or nickel to stretch one fiber arm in a Mach-Zehnder fiber interferometer. When the magnetostrictive material is exposed to a magnetic field, an optical signal proportional to the interferometer phase shift is induced. By calibrating the optical signals to the magnetic fields, the device can then be used as a magnetometer. The characteristics of the magnetometer will depend on the magnetostrictive properties of the magnetic material used. In this paper, the operating conditions relating to sensor sensitivity, linearity and dynamic range of such a fiber magnetometer are presented based on the AC and DC magneto-strictive responses of the sensor material (a metallic glass ribbon). A comparison of experimental results to existing theoretical models is also addressed.

An all-fiber single-mode spectroscopic sensor has been realised. It allows the measurement of the absorption characteristics of a lossy medium. The sensitivi-ty of the device depends on the geometrical factors of the probe and on the refractive index of the sensed medium. Theoretical calculation for a planar geometry and experimental measurements are presented.

The reaction of antibody molecules immobilized onto the sur-face of fused silica fibre optic or planar waveguides with antigens in solution was detected by interaction with the evanescent wave. By detecting in-line fluorescence, the measurement of human IgG is described.

A fiberoptic acoustic sensor constructed in the Mach-Zehnder arrangement for use in a resonant photoacoustic cell is reported. Tests indicate a minimum detectable concentration of 0.5 ppm NOT, in Ar for an input power of 50 mW. This extrapo-lates for 500 mW input to a minimum detectable concentration of 50 ppb.

An optical fibre gas detector for remote monitoring of explosive gases on offshore installations has been developed and tested. The system operates at an absorption line of methane where silica fibres have very low losses.

Fiber optic sensors of physiological parameters are attracting considerable interest especially for invasive measurements,owing to their flexibility,miniaturized size along with electrical insulation and immunity from electromagnetic interference. A review is reported on optical fiber sensors for biochemical and physical parameters measurements.The attention is focalized on the working principles of the most significant devices,in connection with the requirements which are put by the different medical procedures.

The phenomenon of cross-talk between optical fibers is usually used in the development of optical directional couplers. Subject of this paper is the description of a versatile sensor using this phenomenon and its various applications in liquid level sensing, refractometry, thermodletry etc. 111. The sensing element, which is also a part of the transmitting and receiving fibers, can be located either between or at the end of these fibers. The configuration can be chosen according to the type of application and mechanical construction. It is possible to measure the cross-talk signal independent of transmission losses and also to monitor failure of the system due to breakage in the optical or electrical circuit.

The paper describes a system of optical fibre sensors in which the wavelength of the transmitted light conveys information about the state of a transducer.

Optical fibre intensity modulated transducers are attractive in their inherent simplicity and the high sensitivity which can be achieved with relatively simple design. However, they are prone to errors due to variations in the attenuation of feed and return fibre paths, in the optical power levels launched at the receiver and in the sensitivity of the photodetectors. This paper presents a technique whereby all these sources of error may be exactly nulled using an optical analogue to a Wheatstone bridge network. Any spurious readings are then entirely due to deficiencies in the design of the transducer head itself.

A distributed optical-fibre stress sensor with a measurement path length of 25m and a spatial resolution of lm is described. The sensor, the first of its kind, uses the stimulated Raman interaction between counterpropagating pump and Stokes waves.

Plastic optical fibers are now commonly used in lead scintillators for fast particle detection, wheither scintillating or wavelength shifting. Large N.A. fibers from 300 to 2.000 microns 0.D. are commonly drawn from cladded polystyrene doped or undoped core preforms. Various doping will allow the design of entirely new types of sensors.

A high bandwidth system has been developed to measure levels of ionising radiation in pulsed systems using a passive sensor head and an optical fibre cable for remote sensing. Optical fibres offer the advantages of very high bandwidth length products (>1GHz km) and freedom from electromagnetic interference, and allow the use of very small electrically passive sensors. These may be used for example to monitor particle accelerators and US work has been reported where similar systems have been investigated for diagnostics during nuclear events (Refs 1 and 2). Using semiconductor scintillator sources, we have achieved risetimes of 1.8ns at a wavelength of 1.311m, enabling the sensor to operate in a region where optical fibres exhibit low material dispersion and low attenuation. A fast optical receiver, with a risetime of 500 ps has been built to monitor the emitted light. Increases of sensor and receiver speeds are anticipated in our future development programmes.

Many different principles for temperature measurement with fiber-optic sensors have been suggested and brought about experimentally. If we restrict ourselves to the use of sensors employing fluorescence decay time, the first proposal was made by K.A. James et al. (1). They suggest exciting fluorescence by a flash and measuring the intensity of the decaying fluorescent light and determining the point of time at which the intensity has decreased to 1/e of the starting value. Except if laser flashes are used for excitation, measurement according to this principle will surely be very inaccurate as a result of noise. McCormack (2) sinusoidally modulates the exciting light with a fixed frequency and measures the phase shift of the fluorescent light sinusoidally oscillating at the same frequency. The accuracy achievable referred to the intensity of the exciting light is probably better; however, it is difficult to measure any phase shift very exactly.

A thermochromic solution of a Cobalt salt can be used as a temperature sensor,since its optical absorption spectrum is strong modified by temperature variations.A thermometer can be realized making use of fiber optics for light transmission to and from the sensor.Such device has been set up and tested in the laboratory:technical data are here presented and pratical measurements and results are then discussed.

Using a fluoride glass seven-fiber bundle (core diameter 160 μm, length 1.3 m) and a polycrystalline fiber (core diameter 0.5 mm, length 4.8 m), radiometric measurements were made at temperatures above 60°C.

A laboratory version of a novel temperature sensor based on a Fiber Fabry Perot (FFP) interferometer and the wavelength control of a temperature stabilized laser diode is presented. The FFP with finesse of about 10 is observed in its reflected light at a fixed interference order to which the laser wave-length is continuously tuned by a corresponding variation of the injection current. The sensor in its present state allows for a resolution better than 0,1°C and a measuring interval of about 20°C. These properties along with the short and thin sensor tip geometry makes it an attractive device for medical applications such as continuous temperature measurements in hyperthermia systems.

In 1983 the total sensor market was nearly $ 5 billion. The largest segments were flow meters, temperature, level, and pressure sensors. Flow meters are used extensively in the chemical industry, the largest segment of the process control market. Sales of all types of flow meters exceeded $ 1 billion in 1983. Level sensors are also used in chemical plants, but they are also used in the gas and oil transportation segment. Sales of liquid level sensors was $ 500 million in 1983. That same year, sales of temperature sensors was more than three quarters of a billion dollars.

Fiberoptic sensor technology has been developed to satisfy particular needs in specific applications (primarily in the military sector). The technology is now on the verge of commercialization in a broad range of applications in the private sector. As the technology becomes more generic, unforeseen applications will emerge. A review of the patents granted for fiberoptic sensors indicates the range and magnitude of commercial interest in this technology.

We present the first test of a planar integrated optical interferome-trer used as a microdisplacement or vibration sensor. A schematic diagram of this device is shown figure 1. Essentially, it is a Michelson interferometer achieved with our classical planar structure Si/SiO2/Si3N4/SiO2 [1-2] including : . the grating coupler operating in the third order (period = 2μm) ; . the beam splitter, achieved by local etching of the SiO2 over-layer ; . two mirrors also obtained by etching of the silica followed by an aluminium deposition, in order to lower the T.I.R.* angle ; . and two cleaved faces.

A basic system is described, using a mechanically modulated reflected signal and a reference signal generated by photoluminescence for compensation of optical intensity variations. Three sensor designs are presented, two using geometrical effects and one using interferometric modulation of the signal. Furthermore, a novel technique to control the damping of mechanical resonances, is described.

We report a method of fabricating a 45° polarization rotating coupler using single polarization maintaining fiber. We demonstrated that the coupler was useful to localize the sensitivity for temperature sensing of the fiber in a one-fiber interferometer.

A Differential Absorption Displacement Transducer was recently proposed and demonstrated. This transducer was wavelength multiplexed with a ruby glass temperature transducer. Thus the operation of a displacement and temperature multimeasurant transducer was demonstrated.

A new transmission fiber optic temperature sensor is developed from two parallel fibers with a GaAs-platelet between the fiberends. The endfaces are polished at 45 to allow light coupling between the fibers.

An optical fiber liquid sensor for the remote detection was developed. The sensor was confirmed to have the following features;(a) long distance, multiple point detection for more than 10km, (b) insensitive to water, (c) safety in explosive or high voltage environment.

An all-optical remote sensing system utilizing long-distance, ultralow-loss optical fiber networks is studied and discussed for near-infrared absorption measurements of combustible and/or explosive gases such as CH4 and C3H8 in our environment, including experimental results achieved in a diameter More than 20 km.

Input grating couplers on planar SiO2-TiO2 waveguides are extremely sensitive sensors for humidity and other vapours (e.g., ethanol, acetone). Adsorption and desorption increase and decrease, respectively, the effective guide index and, thus, change the incoupling efficiency.

A computer integrated spectral sensor is described that allows simple interfacing with manufacturing and testing devices. Several applications are discussed.

An optical fibre pressure sensor utilizes displacement of a holographic grating to intensity modulate a signal wavelength; a second wavelength passes through the grating unmodulated and its transmittance is used to reference the signal wavelength intensity.

This paper describes, with examples, how optical fibers have raised new concepts in spectranetry, such as the internal spectral reference, instantaneous measurements on the sides of the absorption spectra,and the modelling of spectral variations. With optical fibers, original technical solutions are used for remote chemical analysis.

A differential heterodyne fiber-optic sensing scheme using a transverse Zeeman laser and a birefringence-type sensor cell is constructed which can measure temperature, magnetic flux, voltage and pressure with a precision of 0.04°C, 1G(DC), 0.1V (DC) and 350Pa, respectively.

The contrast of a fiber-optic heterodyne interferometer using 50 μm graded index fibers is investigated as a function of three different fiber optic beam combiners. It is shown that with a beam splitter-type beam com-biner the contrast is 10 dB lower than in a single mode interferometer.

The conventional polarimetric sensor, first devised by Rashleigh and Ulrich (1), is a simple transducer using polarisation maintaining fibre which, utilises changes in the differential optical delay between the two orthogonally-polarised modes of the fibre to sense physical parameters. The relative delay is conveniently observed by monitoring the state of polarisation of the light at the far end of the fibre (fig. 1(a)).

A remote polarimetric sensor configuration is described, and two signal detection schemes which can be utilised in conjunction with the sensor to provide a linearised output are demonstrated.

Fiber optic gyros make use of the Sagnac-effect for rotation rate measurement. Two counterpropagating light beams travelling through a coil with length L and diameter D experience a phase shift 2ΔΦ which is given by (1) 2ΔΦ 2LD Ω λ= wavelength c = velocity of light Ω = angular velocity This phase shift can be detected with a 'phase-nulling scheme' or by direct detection /1/. The direct detection has the advantage of an uncomplicated optical set-up (Fig. 1, /2/), but the signal detection raises some problems. By using an analogue system the linearity and the dynamic range are subject to severe limitations. To overcome these problems we use a digital signal detection scheme.

So far most of the fiber-optic gyros with high resolution and low drift have been realized as all fiber devices /1,2/. Common to these interferometers was the use of phase-modu-lators and of spectrally broad sources. According to Petermann et al. /3/ and to Burns and Moeller /4/ the combination of a spectrally broad light source with a highly birefringent fiber results in an efficient reduction of the short term noise as well as an increase in drift stability. In this paper we report on a fiber-gyro composed of an integrated-optic phase modulator and polarization maintaining fibers. Two light sources with different spectral characteristics were investigated: a superluminescent diode (ELED) and a gain-guided laser (GGL). In both cases a high drift stability was encountered for low power operation. For higher light levels (and thus smaller linewidth) a drift of the output of the gyro occurred, which was found to be caused by reflections from the integrated optic phase modulator. A modified modulation scheme is presented to overcome this problem.

A phase swept pseudo-heterodyne detection scheme suitable for use with all-fibre versions of the fibre gyroscope is described. The output carrier signal, which is phase modulated by the Sagnac shift, is generated by employing sinusoidal phase modulation via a piezoelectric fibre stretcher in combination with selective detector gating.

The polarization properties of the monomode fibers with low and high birefringence were investigated using the radiation sources of various spectrum widths. The gyroscopes on the base of fibers with low birefringence were investigated.

We present a compact all-fiber gyroscope brass-board (0.28 liter) which has a sensitivity of 3 deg/h/ VHz. We also describe improvements that we have brought to all-fiber components, and in particular to couplers using stress-induced polarization - holding fiber.

A digest of designed and performed noncoherent/coherent multicore optical fibre /MOF/ sensors has been presented. The characteristics of interaction of coherent MOF with external physical fields have been calculated theoretically and next verified experimentally.

A review of present and future trends in single mode optical fiber modulators for systems applications is presented. Among the modulators to be discussed are: PZT Cylinder, PZT Resonators, PVF2 jacketed fiber and Quartz Squeezers. Performance characteristics of the different modulators are summarized and compared. System requirements are also discussed.

The paper shows that optical fibre sensors and distributed fibre sensors in particular can be analysed by direct analogy to guided wave radar systems. This shows that all optical fibre sensor measurements can be represented as 4 basic types of measurement corresponding to time and frequency derivatives of the transmission or reflection coefficient of the fibre transmission line. This approach is used to explain and aid the understanding of both distributed optical fibre sensors (OTDR) and passive multiplexing of many fibre sensors. It also indicates the relations between some of the key parameters involved, including source coherence, range resolution, the bandwidth/repetition rate of the associated optical modulation. The particular approach is extendable to various configurations of optical fibre network and is applicable to both single mode and multimode systems.

An optical fiber gyroscope with the integrated optical frequency modulator is studied. and its short time noise level is equivalent to 10 rad/sec. This optical IC includes two geodesic lenses and two SAW modulators on the lithium-niobatre substrate.

Experimental results from an integrated optic fibre gyroscope with on-chip phase modulators are reported. The system has achieved a noise density of 3.5h-1 Hz-0.5 and a standing bias of 700h-l. Bias drift is better than 200h-1 over a ten minute period.

This contribution reports on investigations of the rotation rate error, which occurs at a fibre gyroscope output when its sensing coil is subjected to dynamic temperature variations. The severity of the problem is best illustrated by the fact that the tempera-ture nonuniformity of the sensor coil must be less than 0,01 °C if the system is to resolve rotation rates of 0,01 °/h /1/. This requirement can easily be met in the lab by use of sophisticated control and shielding techniques. However, for a system to be usable under common environmental conditions, special coiling and encapsulating techniques must be developed in order to achieve immunity versus thermal effects.

The polarisation requirements of the resonant-ring fibre gyroscope are analysed. A birefringence-dependent scale factor error and zero stability problem are elucidated. Reduction of these errors using birefringence modulation and differential attenuation are discussed.

The Rayleigh backscattering in Optical Passive Ring-Resonator Gyro (OPRG) is newly formulated in consideration of the temporal coherence of the optical source. This scattering degrades the gyro linearity due to its resonance peak splitting under the rotation. The theoretical limit of rotation sensing is also computed considering the optical source coherence.

Rayleigh backscatter from the sensing coil in an optical fibre gyroscope has for some time been recognised as a source of rotation rate error. This paper describes an analysis which describes the correlation between the backscatter beams in each direction and discusses some of the systems implications of this correlation.

We describe a 1 liter fiber gyroscope brass-board which uses a multi-function integrated optic circuit. Short term sensitivity is 3.5 deg/h/ Hz. We also discuss problems of spatial rejection of double Y junction and polarization-induced non- reciprocity.

When a highly-birefringent fibre is bent and/or tapered, a large differential loss occurs between the two linearly polarised modes, and this effect. can be used to construct high performance polarisers. The present work compares the perfor-mance and limitations of polarisers made from coils and tapers. Taper polarisers with 35dB extinction have been made, whereas coil polarisers have yielded up to 62dB.

Aiming at the design of a magnetic field sensor utilizing the Faraday effect, we give in this paper a description of measurements of magneto-optical properties of YIG. We also give sensor design rules based upon these measurements.

As fiber optic sensors are configured to achieve greater sensitivity and flexibility, compatible low loss laser sources at 1.3 μm and 1.55μm may be required. Similarly, the operation of several sensors from a single multiplexed laser source is also of interest. The high tuning rates achievable with cleaved coupled cavity, C3, semiconductor lasers make these lasers attractive candidates for future sensor applications.1 In this paper, we report the low frequency characteristics of a buried crescent C3 laser operating at 1.3 μm.

The performance of a coherent OTDR system operating at 1.3 μm is described. Two sources have been used - a Nd:YAG laser and a semiconductor laser with increased coherence. With the latter source, greater than 24 dB one way range has been achieved with accurate determination of fault magnitude.

Sensors which rely on the external modulation of the properties of an optical fibre (intrinsic sensors) are receiving much attention since they can be made extremely sensitive, and can be used for distributed measurements. Distributed sensing provides some particularly exciting prospects for acoustic, magnetic and electric field monitoring. To date, however, the great majority of experimental and commercial fibre sensors employ telecommunications-grade fibres, largely as a result of their ready availability. Not only does this policy frequently lead to a design compromise, but in some cases makes the performance marginal or untenable as a result of excessive environmental sensitivity. Despite this, little attention has been given to the design of special sensor fibres with enhanced (or depressed) sensitivity to specific measurands. The position is somewhat better with respect to fibres designed to eliminate sensor polarisation problems (e.g. polar isation-maintaining fibres), but even here further work is required to provide the performance demanded.

Multimode fibers connect two Michelson interferometers, source,and detector. The mirror position of the transmitting interferometer is reconstructed in the receiving interferometer by periodically scanning through three 'white-liciiit' fringes (laser reference, 6 mm range, 0.1 μpm resolution).

The deformation of monomode fibers resulting from a longitudinally applied force has been measured experimentally by means of high resolution heterodyne interferometry and analysed theoretically using second-order theory of elasticity for an isotropic material under finite deformation.

A hydrophone was constructed on reflectometric principles, with an electrically passive underwater section. The sensor consisted of 150 m of fibre, and a lead length of 150 m. Electrical and acoustic characteristics were broadly as expected.

A novel hydrophone array has been reported which measures changes in discrete fibre elements by monitoring the relative phases of returning light pulses from partially reflective splice joints at the extremities of each section. This paper reviews work towards making the array a more practical proposition.

Digital single sideband detection with time discrete phase modulation for interferometric sensors utilizes integrated-optic components, and a direct digital processing. A dynamic range of 57 dB, and a maximum phase error of 0.07° are achieved with 10 bit converters.

The paper describes a remote (intensity based) displacement or pressure sensor. It includes work on a scheme for compensating variations in optical power, receiver sensitivity and fibre transmission loss in addition to tarnishing of the displacement (pressure) diaphragm. Results are presented for a prototype (uncompensated) sensor plus laboratory results for the compensated scheme.

The use of fluorescence decay from is demonstrated as information carrier in a multimode fiber-optic pressure sensor. The fluorescent material properties of rare earth ions in amorphous or crystalline host materials are shown to be suitable for this and similar applications due to the variability and excellent stability of these materials. The design of a prototype pressure sensor system is described, together with experimental results.

A polarimetric fiber sensors stabilized against the ambient temperature change has been developed by cancelling the temperature dependence of the intrinsic fiber birefringence with the thermal stress of a multi-layer plastic jacket.

A method is demonstrated for monitoring the structural integrity of large structures, using an optical fibre. The strain distribution along the structure is monitored by measuring the attentuation of light along the length of the fibre.

The engeneerized prototipe of a fiber-optic sensor useful for vibration measurement in hostile environments is presented.The vibrometer is essentially based on a "differential" heterodyne Michelson-type interferometer in which the reference beam and the sensing beam are both guided, with orthogonal polarization, by the same single-mode polarization-holding fiber. Measurements of large vibration amplitudes of rough metal surface are reported.

This paper describes a novel optical fibre sensing system for three dimensional small mechanical vibration measurements with significantly improved performances over the existing ones. The minimum detectable vibration displacements are on the order of 0.1-0.01 A at frequencies up to 30 MHz.